Omni-directional antennas are widely used in communications for transportation, defense, security, mobile, and other applications. Omni-directional antennas are useful in situations where the direction of another communicating party is unknown, because it is indeterminate how to point the antenna in the specific direction of the other party. Conversely, in radio geolocation (range finding or radio location) where it may be desirable to pinpoint the location of an unknown emitter based on relative power measurements by plural system sensors, each sensor should have equal opportunity to measure the incoming power unskewed by antenna directionality.
In acoustics, 3D-omnidirectional transponders are well known. In contrast, due to the transverse polarization of electromagnetic waves, a true 3D-omnidirectional antenna is impossible. Hereinafter, omni-directional will refer to a simple “doughnut pattern”, which is the characteristic far-field pattern of a small dipole which may be considered as up to a free-space wavelength λ. However, a dipole which is 1.5λ long has a far-field pattern that is azimuthally isotropic, but which exhibits three (3) elevation angle lobes. Adjacent lobes undergo a sign change, implying conical nodes. Unlike the zenith/nadir points of the dipole pattern which are point nodes, the nulls of the far-field pattern are line nodes and present a serious obstacle to 3D power-based geolocation, because an unknown emitter can easily lie in a nodal direction relative to the given sensor. In practice, these nulls can be at least 15-20 dB weaker than the high-gain directions of the antenna, even in an environment free of multipath.
Many broadband antennas exist and are commercially available. However, the commercial terminology “broadband” invariably refers to the impedance behavior of the antenna, or equivalently its return loss or voltage standing wave ratio (VSWR). Essentially, the far-field patterns of such broadband antennas evolve from simple (e.g., dipole-like) at low frequencies, to complicated (multi-lobed or highly directional) at high frequencies. This is especially true for the conventional discone antenna. Another well-known example is the biconical antenna, which has a relatively broadband doughnut-like pattern, but which yields a multi-lobed elevation angle pattern at high frequencies. Additionally, the biconical antenna has a large footprint which may present an excessive wind load outdoors and which may be difficult to construct in an inconspicuous manner for indoor use. Also, broadband biconical antennas may be expensive.
Therefore, there is a need for a compact, ultra-broadband antenna with a simple doughnut-like radiation pattern over a wide operating bandwidth. In particular, elevation angle pattern minima, other than those at the zenith and nadir, should be within 10 dB of the global pattern maximum. In addition, it is desirable that such an antenna be inherently inexpensive.